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2017 East Central NA Regional Contest

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2017-10-28 19:00 UTC

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Problem CDRM Messages

DRM Encryption is a new kind of encryption. Given an
encrypted string (which we’ll call a DRM message), the
decryption process involves three steps: Divide, Rotate and
Merge. This process is described in the following example with
the DRM message “EWPGAJRB”:

Divide

– First, divide the message in half to “EWPG” and
“AJRB”.

Rotate

– For each half, calculate its rotation value by summing
up the values of each character ($\textrm{A} = 0, \textrm{B} = 1, \ldots ,
\textrm{Z} = 25$). The rotation value of “EWPG” is
$4 + 22 + 15 + 6 =
47$. Rotate each character in “EWPG” $47$ positions forward (wrapping
from Z to A when necessary) to obtain the new string
“ZRKB”. Following the same process on “AJRB” results in
“BKSC”.

Merge

– The last step is to combine these new strings (“ZRKB”
and “BKSC”) by rotating each character in the first string
by the value of the corresponding character in the second
string. For the first position, rotating ‘Z’ by ‘B’ means
moving it forward 1 character, which wraps it around to
‘A’. Continuing this process for every character results in
the final decrypted message, “ABCD”.

Input

The input contains a single DRM message to be decrypted. All
characters in the string are uppercase letters and the string’s
length is even and $\leq 15\,
000$.